The present invention is in the field of methods and apparatus for monitoring the operation of, and controlling, a sucker-rod pumping system such as used in the production of crude oil. In particular, the invention provides improved means for detecting incomplete pump filling and broken sucker rods.
In the production of oil, it is common practice to use a reciprocating, positive-displacement downhole pump connected to a surface beam-pumping unit by a string of sucker rods. Vertical reciprocating motion is transmitted from the surface pumping unit to the subsurface pump through the sucker rod string. In most cases the surface pumping unit is driven by an inductive, alternating-current ("AC") electric motor.
A "pumped-off" condition in a rod-pumped oil well occurs when the hydrostatic head--which is related to the height of fluid standing in the well bore--is reduced, thereby reducing the pump's suction pressure. Reductions in hydrostatic head are caused when the well is pumped faster than the rate at which fluid can flow into the well bore from the producing formation. Such reduced inflow pressure prevents the pump from filling completely with liquid, causing the pump to become partially filled with vapor, which results in the creation of a vapor-liquid interface.
As the pump starts on the downstroke when the well is pumped off, the traveling valve fails to open, maintaining the weight of the fluid column on the pump plunger and therefore on the rod string, and through it, on the pumping unit. The pumping system thus becomes "rod heavy," and the plunger travels at an increasing velocity through the vapor space until it strikes the vapor-liquid interface, resulting in a rapid transfer of kinetic energy from the rods to the tubing, a condition known as "fluid pound." Fluid pound can cause damage to surface and subsurface pumping equipment, such as rods, pump, or tubing.
Fluid pound occurs when the well is pumped off. Therefore, to prevent damage associated with fluid pound and to maintain production efficiency, it is desirable to stop the pumping system when pump-off occurs, to allow time for fluid to refill the well bore.
Should the rod string break--another frequent problem in pumping wells--the pumping system will be placed in a condition of imbalance. The system becomes "counterweight heavy" due to the loss of the fluid load supported by the rod string. Continuing to operate in the unbalanced condition that results from a broken rod not only is highly inefficient (in that no liquid is being produced) but also may result in damage to the surface equipment. It is desirable to stop the operation of the pumping system in such a circumstance and to activate an alarm, to alert the operator that a problem exists.
A number of pump controllers have been produced that employ rod load and displacement measurement to detect a pumped-off condition automatically. Each of the following examples of pump-off controllers examines the load on the pumping equipment in relation to at least one position in the pumping stroke:
Boyd et al. teach in U.S. Pat. No. 3,306,210 comparison of rod loading at a predetermined point in the downstroke to a predetermined load value, and use mechanical means for the detection and control of pump-off.
Standish teaches in U.S. Pat. No. 4,286,925 comparison of rod loading at a predetermined position in the downstroke to a predetermined load value, and uses electronic means for the detection and control of pump-off.
McTamaney et al. teach in U.S. Pat. No. 4,487,061 comparison of rod loading at a predetermined position in the downstroke to a predetermined load value, also using electronic means for the detection and control of pump-off.
It is common practice to calculate the work done by the rod string by integrating the polished rod load with respect to polished rod displacement for one complete stroke of the pumping unit. Each of the following examples of pump-off controllers examines the work being done by the pump for all or a portion of the pumping stroke:
Gibbs teaches in U.S. Pat. No. 3,951,209 detection of a decrease, between the full-pump and pumped-off condition, in the integration value of the entire area within the dynagraph, which is a chart produced by measurement of the rod loading and rod displacement as a means for detecting and controlling pump-off.
Womack et al. teach in U.S. Pat. No. 4,015,469 detection of a decrease, between the full-pump and pumped-off condition, in the integrated area within the upper-stroke portion of the dynagraph as a means for detecting and controlling pump-off.
Montgomery et al. teach in U.S. Pat. No. 4,583,915 detection of an increase, between the full-pump and pumped-off condition, in an integrated area below the downstroke load measurements as a means for detecting and controlling pump-off.
Montgomery et al. teach in U.S. Pat. No. 5,224,834 detection of an increase, between the full-pump and pumped-off condition, in an integrated area comprising a part of the downstroke.
A number of other pump-off controllers have been produced that detect pump-off by identifying changes in the current supplied to the induction alternating current electric motor, which drives a cyclically operated walking beam pumping unit to raise and lower the subsurface pump by means of a string of sucker rods. Each of the following systems measures the relative amount of energy supplied to the motor during several pumping strokes or a portion of a pumping stroke:
Hubby teaches in U.S. Pat. No. 3,440,512 detection of an increase, from the full-pump to the pumped-off condition, in the difference in peak values of motor current between an upstroke and a subsequent downstroke as a means for detecting and controlling pump-off.
McKee teaches in U.S. Pat. No. 3,953,777 detection of a decrease, between the full pump and pumped-off condition, in the average current drawn by the motor over a period of greater than one pumping stroke as a means for detecting and controlling pump-off.
Welton et al. teach in U.S. Pat. No. 4,058,757 detection of the reduction, from the full pump to the pumped-off condition, in the integrated value of motor current drawn during a portion of one pumping stroke as a means for detecting and controlling pump-off.
Schmidly, Jr. teaches in U.S. Pat. No. 3,509,824 detection of the reduction, between the full pump and pumped-off condition, in electrical power consumed by the motor during the down portion of the pumping stroke as a means for detecting and controlling pump-off.
In addition, Tibbetts et al. teach in U.S. Pat. No. 4,420,787 a process of detecting reduced loading of a motor, but there is no apparatus disclosed for selecting part of a pumping stroke, and the Tibbetts process is not suitable for use with a reciprocating pump stroke.
In general, prior art systems are not entirely satisfactory, because they generally require expensive and complex, and in some cases unreliable, components. Those systems requiting the measurement of polished rod load and displacement are expensive to purchase, install, and maintain. Those systems relying on measurement of motor current require careful sizing of measuring elements and are subject to errors from fluctuations in line voltage. In addition, prior art systems typically require manual calibration of the logic for a specific well, in part because the algorithms may vary depending on the capacity of the pumping equipment.